Some Windows Server 2003 or XP users may still be using IE6 - so particularly for Silverlight 1.0 or any interactions with the page DOM you'll need to test your code. Easiest thing to do is to download the appropriate VPC image to get access the additional browsers you need:
http://www.microsoft.com/downloads/details.aspx?FamilyId=21EABB90-958F-4B64-B5F1-73D0A413C8EF&displaylang=en